1. Field of the Invention.
The present invention generally relates to apparatus for the direct conversion of radioactive decay energy to electrical energy without going through a thermal cycle, and more particularly, it relates to semiconductor power cells which convert ionizing radiation directly into electrical energy.
2. Related Art
The decay of radioactive materials produces electrically charged particles such as alpha and beta particles as well as photons such as gamma rays. As with other nuclear processes, the charge scale of these types of radiation is millions of times greater than in non-nuclear processes.
For example, isotope Americium-241 has a half-life of 458 years and produces alpha decay which can introduce 5.5 million electron volts (MeV) into a typical ionizable material. On the average, only 3.6 electron volts (eV) are necessary to produce one electron-hole pair within the typical semiconductor material. Thus, for every alpha particle traveling through a semiconductor material, approximately 1.5 million electron-hole pairs may be formed by the absorption of the single alpha particle. This phenomenon of many electron-hole pairs for each particle is known as the multiplication factor for power cells based on radioactive decay. In contrast, for a typical photo-cell, having no multiplication factor, each photon that is absorbed by a photon-responsive semiconductor material generates only one electron-hole pair.
Heretofore, there have been known several methods for conversion of radioactive energy released during the decay of natural radioactive elements into electrical energy. To date, the only type of device with practical application has been the so-called Radioisotopic Thermoelectric Generator (RTG), utilized primarily by NASA in space-based applications. Other technologies have been explored, including the pn-junction type converter, and are currently under development by several laboratories, but none of the other technologies have yet seen substantial commercial application.
U.S. Pat. No. 5,087,533 entitled "Contact Potential Difference Cell" issued to Paul Brown on Feb. 11, 1992, discloses an isotopic electric converter comprised of dissimilar metallic electrodes separated by a semiconductor medium. Said semiconductor medium has a radioactive isotope dispersed throughout. Radioactive decay produces ions in the semiconductor that are swept away by the contact potential difference, which is a result of the difference in work functions of the two dissimilar metals. This device is relatively bulky and inefficient due to the metallic electrodes, and recombination within the semiconductor, together with space charge effects, limit its efficiency, resulting in relatively low power density.
U.S. Pat. No. 5,246,505 (Patent '505) entitled "System and Method to Improve the Power Output and Longevity of a Radioisotope Thermoelectric Generator" issued to Alfred Mowery, Jr., on Sep. 21, 1993, discloses an apparatus which provides electrical power by utilizing the waste heat generated by a large amount of radioactive material, typically plutonium. In the process of Patent '505, thermocouples placed about the heat source convert said heat energy into electricity. Helium outgassing is used for cooling to reduce thermal degradation effects, which yields a greater working life for the apparatus. However, all the problems inherent to the RTG design are still present, namely, the apparatus is large and massive and works with great inefficiency, and substantial shielding is required because the radioisotope employed is of great health risk.
U.S. Pat. No. 5,280,213 (Patent '213) entitled "Electric Power Cell Energized by Particle and Electromagnetic Radiation" issued to John Day on Jan. 18, 1994. Patent '213 discloses a secondary emission type power cell wherein ionizing radiation is attenuated by a secondary emissive material (a material that emits secondary electrons) in order to generate slow-moving secondary electrons for charging metal capacitor plates separated by dielectric material. This type of apparatus does exhibit a multiplication factor, but is limited by the use of the dielectric material to pulsed operation. Recombination within the secondary emitter limits the efficiency of the apparatus.
U.S. Pat. No. 5,605,171 (Pat. '171) entitled "Porous Silicon with Embedded Tritium as a Stand-Alone Prime Power Source for Optoelectronic Applications" issued to Shiu-Wing Tam on Feb. 25, 1997. Patent '171 discloses a radioluminescent apparatus coupled with a photovoltaic cell, wherein radioactive decay energy is converted into light energy that is then, in turn, converted into electricity by a solar cell. Although these types of cells are fairly reliable, their efficiency is severely limited by the indirect method used for the energy conversion.
U.S. Pat. No. 5,606,213 entitled "Nuclear Batteries" issued to Nazir Kherani et al. on Feb. 25, 1997 and discloses a nuclear battery made by incorporating radioactive tritium in a body of amorphous semiconductor material. An electropositive conductivity region and an electronegative conductivity region, with a semiconductor junction therebetween, are provided in the semiconductor material, with the electropositive and electronegative regions being connected to a load circuit. This type of cell lacks stability, however, and soon loses its powergenerating effectiveness, which loss is attributed to dangling-bond degradation.
U.S. Pat. No. 5,616,928 entitled "Protecting Personnel and the Environment From Radioactive Conditions by Controlling Such Conditions and Safely Disposing of Their Energy" issued to Virginia Russell on Apr. 1, 1997. This patent discloses a primary charging apparatus wherein a radioactive source is enclosed within a body consisting of metal plates. The metal plates are separated by dielectric material, forming a capacitor housing that is charged by the decay particles. Primary cells based on this disclosed invention will operate at very high voltage and in a pulsed manner. Also, their space charge effects, as well as reverse leakage currents, will limit their efficiency for power generation.
U.S. Pat. No. 5,642,014 entitled "Self Powered Device", issued to Steve Hillenius on Jun. 24, 1997, discloses a pn-junction type of isotopic electric converter provided with an integrated circuit powered by said converter. The apparatus is built of a layer of a first conductivity type and a layer of a second conductivity type to form a pn-junction depletion layer. A tritium-containing layer is provided, which supplies beta particles that penetrate the depletion layer. The penetration generates electron-hole pairs that are swept by the electric field in the depletion layer, producing an electric current. However, the problems inherent to the pn-junction type of cell are that the junction is a fragile crystalline structure and constant bombardment of beta particles causes material degradation effects. The degradation effects destroy the junction and limit its useful life, and also limit the upper power availability of this type of apparatus. Annealing or hardening of the junction has been employed to reduce the effects and provide greater operating life from such cells, but the problem of material degradation still remains.
Each of the above-cited U.S. patents discloses apparatus and means for converting radioactive decay energy into electricity, yet none of the designs has seen substantial commercial application due to the shortcomings of each design. All isotopic electric converters, no matter what type, are actually electric generators fueled by radioactive decay energy. Since that decay energy is fairly constant, the electrical output from such an apparatus is fairly constant and no means is provided for increased power demand for peak operations, such as the start-up of the electrical load. On the other hand, an electrical energy source constructed according to the principles of the present invention does not suffer the performance and efficiency limitations of the prior art.